To investigate the process temperature on the growth of ultra-thin (≤500 nm) Cu(In,Ga)Se2 (CIGSe) absorbers and the corresponding performance of solar cells, the process temperature was set to 610 °C and 440 °C, respectively. It was found that the low process temperature (440 °C) could reduce the inter- diffusion of Ga–In and thus result in a higher back [Ga]/([Ga]+[In]) ([Ga]/[III]) grading than at the temperature of 610 °C. The higher back [Ga]/[III] grading at 440 °C was evidenced to both electrically and optically contribute to the efficiency enhancement of the solar cells in contrast to the lower back [Ga]/[III] grading at 610 °C. It was also implied that the high back [Ga]/[III] grading was beneficial to the collection of carriers generated from the back-reflected light.
